If no more water flowed into the lake from this moment and it was drained down to 909 feet , that would be 504,000 acre feet of water from this storm down the Guadalupe to the Gulf.   In perspective, that is 164.3 billion gallons or enough water to supply 1,008,000 families of 4 with water for a year.  That is water for 4,032,000 million people for a year.  (At least this much more water flowed through Canyon Lake in the following days.)

While the peak flows of past storms have not been reached I think this flood will set a record for duration and total volume of water, considering the flow in the Medina River as well.

Is the Dam Safe?

The lake can hold 1,129,300 acre feet and I presume that is at the top of the dam.  At that elevation the spillway would discharge 502,800 cfs.  Looks like there is an adequate factor of safety on the dam capacity that Mother Nature will be hard pressed to beat.   For comparison, the USGS record shows a peak flow past New Braunfels in 1998 of nearly 150,000 cfs and the maximum inflow to the lake was 160,000 cfs in 1978.  The average flow that day was about 75,000 cfs.

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Criticism of the Corps of Engineers for not releasing water from Canyon Lake before water went over the spillway, was printed in the San Antonio paper and in letters to the CLTG.    When it was noted that by law the Corps could not release water when the flow at Gonzales was over 12,000 cfs (cubic feet per second), one Canyon Lake resident was quoted as suggesting that the Corps should have broken the law. 

For the record, the flow at Gonzales exceeded 12,000 cfs on July 2 at 10:00 pm   and dropped below 12,000 cfs on July 11 around midnight.  On July 4, 2002 when water just started to go over the spillway in Canyon Lake the flow at Gonzales was already 40,000 cfs.

We decided to check out what the effects would be had water been released at a steady rate between July 1 and July 4 when the water went over the spillway –  a period of about 100 hours.

We did an analysis of the effect of releasing 5000 cfs constantly from the time the lake level went above the conservation pool elevation of 909 feet and plotted the results on a graph.  The dashed curve on the graph is the approximate manner in which the lake would fill under that scenario.  The solid curve is the actual lake level.  To confirm the dashed curve, 5000 cfs over 100  hours is about 40,000 acre feet.  If you take 40,000 acre feet out of the lake just as it reached the spillway at elevation 943 feet, the lake level would drop over 3 feet and require another 10 hours of the rate of inflow at that time to reach the spillway.  That corresponds with the end point on the curve.  More details may be found at http://hillcountrywater.org under “Stream Flow Data”

Letting water out as the lake level rose for this particular flood would have made little difference in the magnitude of the flood but would have shortened the time the river was at its peak below Canyon Lake by about 10 to 12 hours.   However, releasing another 5000 cfs to the river while Gonzales was above flood stage was simply not possible.

The red curve was generated on an hourly basis which was tedious because the volume change in the lake is different for each foot of elevation change as the lake rises.  From elevation 909 to 910 the volume increases 8436 acre feet.  From elevation 942 to 943   the volume increases 12,800 acre feet.

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